This invention relates generally to automatic disconnect systems for telephone circuits, and is particularly directed to a system for disconnecting a telephone circuit in response to a "hang up" by one party on a telephone line.
In the type of telephone system under consideration, a telephone line and intermediate switching circuitry couple a calling party's line to a called party's line. Generally, the system also includes means for detecting when one of the party's lines goes "on-hook" so that his telephone can be disconnected and made ready to receive or initiate further calls. Typically, the disconnect occurs approximately twenty seconds after the "on-hook" condition is detected.
Recently, apparatus for automatically recording messages from a caller and for transmitting recorded messages to a customer or client have been introduced. When a party is in communication with such apparatus, it is highly desirable that the customer's telephone line be quickly disconnected when he hangs up, i.e., goes "on-hook". For example, if the customer does not desire to hear the recorded message, he must have the ability to hang up and have his telephone freed or disconnected in order to make and receive further calls.
In the past, disconnect circuitry has typically sensed the hang-up click generated when a party hangs up, and based on that click, a disconnect was effected. Typical of such circuitry is that described in U.S. Pat. No. 3,676,605.
One of the assumptions that such prior art circuitry relied on was that the hang-up click was greater in amplitude than the voice signals. Thus, when a signal greater in amplitude than voice signals was sensed, that signal was used to trigger the disconnect. Of course, when a signal other than a hang-up click exceeded the expected amplitude of voice signals, the disconnect was improperly effected.
In modern telephone systems using electronic switching systems, the hang-up click is not invariably greater in amplitude than the voice signals. In fact, it is usually lower. Hence, amplitude discrimination alone is no longer effective for sensing a hang-up click.
Further, automatic message transmitting/receiving equipment is frequently used in connection with a local operator who initiates a call to a customer, places the apparatus on line with the customer, and then hangs up. Thus, the customer will be in communication with the automatic message transmitting equipment and the local operator is free to initiate another call.
Obviously, any hang-up click generated when the local operator hangs up must be distinguished from the hangup click of the customer. Otherwise, the customer's line would be disconnected when the local operator hangs up. The type of prior disconnect circuitry discussed above is generally not capable of reliably making that distinction.